It is known that the exhaust system of an internal combustion engine can be equipped with one or more after-treatment devices. Such an after-treatment processing device may be any kind of device that is configured so as to modify the composition of the exhaust gases. Some examples of after-treatment devices include NOx traps for lean operation (LNT) and particulate filters (DPF).
In particular, the NOx trap for lean operation as an inexpensive alternative to a system for selective catalytic reduction (SCR) is an after-treatment technology for reducing NOx emissions from an engine based on a NOx storage capacity during the Diesel engine standard working condition, that is to say in lean combustion phases. The NOx is then reduced during a rich combustion phase (DeNOx regeneration). The rich combustion phase is induced by a special control of the air and fuel supply.
Both LNT and DPF controllers use specific models in order to control these devices. The models need several inputs, the most important of which for correct functioning is the quantity of oxygen in the exhaust line.
Such an oxygen quantity signal is typically provided by an air/fuel ratio sensor (also called an oxygen sensor). Depending on the after-treatment configurations, it is possible to have a single oxygen sensor disposed upstream of the LNT or two oxygen sensors one upstream and another downstream of the LNT.
Exhaust gas oxygen sensors are used regularly in diesel applications to control fuel injection in order to remain in compliance with emission standards during the operating life of the vehicle and statutory requirements concerning on-board diagnosis (OBD II) regarding system faults in the fuel injection system. In particular, the oxygen signals are used to detect injector drift and balance the system setting and pilot injection deviation, thereby reducing combustion noise and ensuring good emissions performance. In some applications, oxygen measurement is also used to control and monitor the regeneration event of the NOx trap for lean operation.
In such a case, modulation of the post-injections is extremely critical during the rich combustion phases using a standard step target value adjustment for the oxygen sensor, especially if the sensor is functioning more slowly than the nominal condition due to possible deposits of hydrocarbons (HC) and soot on the probe. The transition from a lean to a rich combustion phase takes place in a sudden step change in the air/fuel ratio (AFR) target value, e.g., from 1.5 to 0.95. The standard adjustment of the oxygen sensor is made on the basis of proportional-integral (PI) control, which acts on the post-injection quantity in conjunction with the AFR target value. The standard control causes the ratio to fall below the AFR (AFR values as low as 0.9 are achieved, for example), which in turn results in an increase in the hydrocarbon quantity, thus contributing to soiling of the sensor probe. This phenomenon slows the sensor performance down, and the situation is aggravated further by the ageing of the sensor.
Therefore, there is a need for a method to control the oxygen concentration, with which the drawbacks described above may be overcome. In one aspect, the method is embodied as a computer-implemented method.